Certain V4L2 fourccs don't (yet) have DRM counter parts, in which case
we can't create DMA_DRM caps for them. This is usually the case for
specific tilings, which are represented as modifiers for DMA formats.
While using these tilings is generally preferable - because of e.g.
lower memory usage - it can result in additional conversion steps when
interacting with DMA based APIs such as GL, Vulkan or KMS. In such cases
using a DMA compatible format usually ends up being the better option.
Before the addition of DMA_DRM caps, this was what playbin3 ended up
requesting in various cases - e.g. prefering NV12 over NV12_4L4 - but
the addition of DMA_DRM caps seems to confuse the selection logic.
As a simple and quite robust solution, assume that peers supporting
DMA_DRM caps always prefer these and reorder the caps accordingly.
In the future we plan to have a translation layer for cases where
there is a matching fourcc+modifier pair for a V4L2 fourcc, ensuring
optimal results.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6645>
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